This invention relates to a process for the production of a homogeneous ream-free body made of quartz glass or made of a glass having a high content of silicic acid, by the shaping of an essentially bar-shaped initial body in a shaping step, the initial body being twisted about its longitudinal axis to form a twisted body radially homogenized in layers having axial layering.
A process of this kind is known from German Patent Specification 567,863. In the known process, a quartz-glass cylinder is held in a heating space between two bars fused to the cylinder ends and is twisted about its longitudinal axis as a result of relative movements of these bars in relation to one another. By the twisting of the cylinder, the glass mass is intermixed in the radial direction, and existing inhomogeneities, which can be detected, for example visually, as reams, are at the same time essentially removed. However, as seen in the direction of the cylinder axis, no intermixing takes place during the twisting, so that inhomogeneities along the cylinder axis persist. As a result of a twisting of this type, therefore, a kind of layered structure of reams is produced in the cylinder, the layers extending perpendicular to the cylinder axis and causing optical faults, depending on the direction in which the cylinder is subjected to stress. For the complete homogenization of the glass mass, the known process proposes kneading and agitation by means of fused-on bars until virtual freedom from reams is achieved.
By means of this process, homogeneous bodies made of quartz glass, having a weight of up to approximately 8 kg, can be produced. With an increasing weight of the glass mass, however, problems regarding the retention of glass mass by means of the bars and a sufficiently uniform heating of the body over its entire cross section also become increasingly evident. In particular, where heavy glass masses are concerned, pronounced mixing and shaping movements, such as occur, for example, during agitation and kneading, cannot be executed as a result of the mechanical stress on the points of fusion to the glass mass.
A process for the production of rods made of bubble-free quartz glass in a single-stage operation is known from German Offenlegungsschrift 3,201,777. In this process, a rod is drawn off continuously from a quartz-glass cylinder heated at one end, the cylinder and the rod being rotated in opposition about their longitudinal axis, and the glass mass being twisted in the heating zone. However, as a result of the absence of intermixing in the direction of the longitudinal axis, the production of quartz-glass bodies which are homogeneous and ream-free in all directions in space is not possible by means of this process.
U.S. Pat. Specification No. 4,358,306 describes a process for the forming of a quartz-glass block, in which an initial block is heated in a graphite mold, deformed by means of a pressure ram lowerable into the graphite mold, and subsequently cooled slowly. The cooling operation is intended to prevent reams or cracks from occurring. However, because of the absence of intensive intermixing of the quartz-glass mass, a completely homogeneous body, particularly ream-free over relatively long distances, cannot be produced by means of this process.
The object on which the present invention is based is to provide a cost-effective process for the production of homogeneous large-volume bodies made of quartz glass or made of a glass having a high content of silicic acid.